The proteome may be reflective of the physiological and pathological states of a subject. Proteomics is a powerful tool for diagnostics of diseases and monitoring of therapeutic responses. The majority of current protein assays in clinical settings are based on enzyme-linked immunosorbent assay (ELISA) immunoassays, which may require high-quality antibodies and may be hard to achieve with high multiplexing (e.g., greater than 10) due to the cross-reactivity of antibodies. Mass spectrometry (MS) measures the mass-to-charge ratio of charged species and may be an enabling technology for proteomics. Aside from de novo identification of target proteins, MS has advantages over ELISA for detecting protein mutations, modification, truncations, and adductations, for example, once combined with the liquid chromatography (LC), liquid chromatography mass spectrometry (LC-MS) may enable separation, identification, characterization, and quantitation of complex mixtures of proteins and peptides and their isoforms.